Overlay alignment measurement of wafers

ABSTRACT

The present invention is a new target, and associated apparatus and methods, for determining offset between adjacent layers of a semiconductor device. The target disclosed here includes a first periodic structure to be placed on a first layer of the device and a second periodic structure, that complements the first periodic structure, placed on a second layer of the device at a location that is adjacent the first periodic structure when the second layer is placed on the first layer. Any offset that may occur is detected by the present invention either optically, micro-mechanically or with electron beams using any of various methods and system embodiments.

FIELD OF THE INVENTION

The present invention relates to th measurement of the alignment of twosuccessive layers on a semiconductor wafer during the production of thewafer. More specifically to the use of a new alignment pattern andcorresponding measurement instruments to determine the registrationaccuracy between the two successive thin film layers on a semiconductorwafer.

BACKGROUND OF THE INVENTION

One of the most critical process control techniques used in themanufacturing of integrated circuits is the measurement of overlayaccuracy between successive layers on a wafer (i.e., the determinationof how accurately a layer aligns with respect to the layer below it).

Presently this measurement is done with patterns that are etched intothe layers. The relative displacement of the two layers is measured byimaging the patterns at high magnification on an electronic camera andby calculating the image position on the camera using any of a varietyof known image analysis algorithms. The most commonly used patterns areconcentric squares with dimensions of approximately 20 micrometers oneach side, generally referred to as "box within a box" target. Thisprior art is described and analyzed by Neal T. Sullivan, "SemiconductorPattern Overlay", in Handbook of Critical Dimensions Metrology andProcess Control, pp 160-188, vol. CR52, SPIE Press (1993).

The resulting accuracy of the prior art is therefore limited by theasymmetry of etched line profiles, by aberrations in the illuminationand imaging optics, and by image sampling in the camera.

It would be desirable to have a system that overcomes the limitations ofthe prior art. It is the intent of the present invention to rectify theshortcomings of the prior art in several areas:

1. The present invention illuminates and collects the light over a verynarrow field of view, so it does not suffer from off-axis aberrationspresent in the prior art;

2. The present invention samples the signal at a rate that is as high asnecessary to meet the required accuracy (e.g., higher than 1000 samplesper micron), while the prior art sampling rate is low (typically lessthan 10 samples per micron), and limited by the number of pixels in thecamera;

3. The alignment requirements for the present invention are loose (onthe order of 5 to 10 microns in displacement, and 10 degrees in angle),while the prior art requires high alignment accuracy for repeatablemeasurements (e.g., 0.1 micron in displacement, 0.01 degrees in angle);and

4. The prior art requires very precise focusing to perform themeasurement (about 0.1 micron).

The present invention does not require such focusing accuracy.

SUMMARY OF THE PRESENT INVENTION

The present invention uses a new target pattern, and method andmeasuring instrument. The target of the present invention is composed ofalternating periodic structures on two successive layers of asemiconductor wafer to provide alignment information. The targetstructures of the present invention are produced by lithographictechniques, and in the simplest implementation includes etched, ordeposited, parallel lines with the inter-line spacing being the same asthe line width, forming periodic gratings. Alignment information is thenobtained by scanning one or more light beams across the two alternatinggratings, generating a pair of detected reflected signals representativeof the variations in the reflected light from the scanned beams, then bymeasuring the relative offset, or phase shift, between the detectedreflected signals the alignment of the layers can be determined.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1a illustrates a typical prior art "box in a box" target pattern.

FIG. 1b illustrates an embodiment of the periodic grating of the presentinvention used to determine the alignment of two layers of asemiconductor.

FIG. 2 is a block diagram of an embodiment of an alignment scanner ofthe present invention to measure the alignment of the layers of asemiconductor wafer.

FIG. 3 illustrates an embodiment of an alignment measurement system ofthe present invention using a periodic grating of the present inventionthat is perpendicular to the y-axis with an instantaneous position oftwo light beams used for measurement of alignment in the y-direction.

FIG. 4 displays in time and position the relationship of the signalsdeveloped by each of the scanned light beams in FIG. 3.

FIG. 5a illustrates another embodiment of an alignment measuring systemof the present invention that utilizes time-division multiplexing.

FIG. 5b illustrates the waveforms produced by the second embodimentalignment measuring system of FIG. 5a.

FIG. 6a illustrates a further embodiment of an alignment measuringsystem of the present invention wherein the light beam sources aremultiplexed spectrally.

FIG. 6b illustrates an embodiment of the alignment measuring system ofthe present invention wherein the light beam sources are detectedadjacent detector elements.

FIG. 7a illustrates a modified embodiment of an alignment system of thpresent invention wherein the light beam scanning is performed opticallyby means of a rotating mirror.

FIG. 7b illustrates a time-division multiplexed implementation with asingle light source and two movable mirrors.

FIG. 8 illustrates a scanning electron microscope implementation of thepresent invention.

FIG. 9 illustrates an embodiment utilizing atomic force detectiontechniques to implement the present invention.

FIGS. 10a and 10b more specifically illustrate an atomic forcemicroscope embodiment of the present invention.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present art of alignment measurement uses a "box in a box" typetarget 5 that is typically like the one illustrated in FIG. 1a. Innerbox 1 is typically printed on the top layer of the semiconductor waferbeing produced, while the open-center-outer block 2 is printed on thesecond layer down on the semiconductor wafer. The measurement processthus involves imaging of target 5 on an electronic camera, by means of amicroscope system, at a high magnification (1000×, typically) and withhigh resolution in both x and y directions.

The registration error in each of the x and y axes is measured by firstcalculating the locations of the edges of lines c1 and c2 of the outerbox 2, and the edge locations of the lines c3 and c4 of the inner box 1.From those locations the misregistration between the two boxes isdetermined by comparing the average separation between lines c1 and c3with the average separation between lines c4 and c2 (i.e., themisregistration between boxes 1 and 2 is the difference between thosetwo separations). The misregistration between boxes 1 and 2 in each axisis thus calculated using the following formulas:

    R.sub.x =(c.sub.x 3-c.sub.x 1)-(c.sub.x 2-c.sub.x 4)       (1a)

and

    R.sub.y =(c.sub.y 3-c.sub.y 1)-(c.sub.y 2-c.sub.y 4)       (1b)

Thus, if the average spacing between lines c1 and c3 is the same as theaverage spacing between lines c2 and c4, the corresponding value of R inthat axis will be zero.

The best practical optical resolution is approximately 1 micron, thusimage processing algorithms are needed to estimate the precise locationof the boxes, so that the required accuracy of better than 0.01 microncan be obtained.

The present invention, in each of the various embodiments, uses a targetthat is composed of periodic structures on each of two consecutivelayers of a semiconductor device to provide relative positioninformation between those two layers of the semiconductor device. Thosestructures are produced by suitable lithographic techniques, and in thesimplest application constitute etched or deposited lines of an equalwidth to the distance between the lines, forming a periodic grating asshown in FIG. 1b and discussed in more detail below. One embodiment ofthe periodic structures (see FIG. 1b) consists of equally spaced lineshaving essentially a square profile with the lines arranged so thatthere is no overlap between the portions of the periodic structurecontributed by each of the two layers. The lines from each layer of thesemiconductor device appear side by side in the periodic structure.While a square profile is preferred because it is easily manufactured bylithography means, it is not essential for operation, and other lineprofiles, trapezoidal or rounded, could alternatively be used.Additionally, while the patterns for both the x- and y-directions areshown side-by-side here, they could be in different locations on thesemiconductor wafer if space is limited. This configuration contributesto various embodiments of a comprehensive measuring instrument foroverlay alignment of semiconductor layers that are discussed below.

FIG. 1b shows a periodic structure in the form of alignment pattern 10,shown here in perfect alignment between the layers which will becomeclear from the following discussion. Alignment pattern 10 includes twoidentical grating patterns, 20 and 30, translated 90° with respect toeach other. Given the axis orientation markings in FIG. 1b, left grating20 is for x-axis registration measurements, while right grating 30 isfor y-axis measurements since the lines of the respective grating mustbe non-parallel to the axis of the measurement to be effective. The usercan choose any convenient orientation of the target relative to theplacement of the dies on the wafer with that orientation being thesame-for the masks from layer to layer. Otherwise the orientation of thetarget and the die patterns on the masks is not critical.

Each of the left and right gratings 20 and 30, as shown in FIG. 1b,consist of four sets of solid parallel line segments with the width ofeach line equal to the spacing between the line segments. In actual usethe configuration of line-width to spaces will vary, depending on therequired accuracy, however that relationship will typically be theminimum linewidth on the semiconductor wafer.

Directing attention to the left grating 20 for illustration, the linesegments 40a and 40b, shown in solid outline, are etched on one layer ofthe semiconductor wafer, while the line segments 50a and 50b, shown insolid outline (solid in reality), are etched on the subsequent layer ofthe semiconductor wafer. Additionally, line segments 40a and 50a arefull length line segments that are the outer line segments of thegrating contributed by each of the semiconductor wafer layers, while thesubstantially half length line segments 40b and 50b make up the centralregion of the periodic grating with each set contributed by a differentlayer of the semiconductor wafer. In right grating 30, line segments 60aand 60b are shown corresponding to, and being on the same layer of thesemiconductor wafer as, lines 40a and 40b in left grating 20. Similarly,in right grating 30, line segments 70a and 70b are shown correspondingto, and being on the same layer of the semiconductor wafer as lines 50aand 50b in left grating 20. This is for illustration here and need notbe matched that way in actual use (i.e., line segments 40a, 40b, 70a and70b might be on the same layer, while line segments 50a, 50b, 60a and60b might be on the other layer).

Note, that in FIG. 1b five lines of each length are shown in eachgrating being contributed by each layer of the semiconductor wafer. Thenumber of lines that are used is dependant on the resolution requiredand the signal-to-noise ratio that must be overcome. From theperspective of the minimum number of each length of lines that is neededfor operation, that number is two "a" and two "b" lines beingcontributed by each of the two consecutive layers of the semiconductorwafer for each of left and right gratings 20 and 30, respectively. Noteadditionally, that if the lines shown in solid outline are printed onthe first layer of the semiconductor wafer with the lines shown solid onthe second layer, then on the third layer another set of lines (shownhere in solid outline) are printed over, and covering, the region oflines 50a and 50b of the first layer. Then the lines 40a and 40b of thesecond layer are used in conjunction with lines 50a and 50b on the thirdlayer. Thus, each set of lines on a layer of the semiconductor wafer(except for those on the first and last layers) are used in conjunctionwith the lines on two layers of the semiconductor wafer, the one belowand the one above. Alternatively, if there is sufficient space on thesemiconductor wafer surface, the grating pairs for each pair of adjacentlayers on the wafer could be in a different location on the wafer tominimize any "bleed through" interference from a third layer on themeasurement for the top two layers of interest.

Thus, given that configuration, any offset between the two layers of thesemiconductor wafer in the left grating 20 will be present between thejuxtaposed lines of 50a and 50b and 40a and 40b. Similarly, in the rightgrating any offset will be present between the juxtaposed lines of 60aand 60b and 80a and 80b.

FIG. 2 is a block diagram of one embodiment of an alignment scanner ofthe present invention that utilizes an alignment pattern, for examplethe alignment pattern 10 discussed with respect to FIG. 1b. In thisembodiment, wafer 100 with the pattern thereon being measured is placedon stage 200 which is typically motor driven under the control of systemcomputer 190 with scan head 230 provided to perform the actualmeasurements with computer 190 also performing the actual calculationsfrom the data received from scan head 230. There are two options withrespect to providing scanning motion between wafer 100 and scan head230. One is to move stage 200 relative to scan head 230 by computer 190,and the other is to move scan head 230 by computer 190 via scan actuator240 (e.g., a piezo electric actuator) relative to stage 200. Whileeither technic can be used, it is preferred to move scan head 230 andhold stage 200 stationary since the scan head can be made much smallerin size and weight relative to the wafer positioning stage.Alternatively, scanning can be implemented by moving the whole head, orby moving only some of the optical components.

Before proceeding with the discussion of the construction and operationof the first embodiment of the present invention shown in FIG. 2, thereare a few definitions to be addressed. In FIG. 2, and the figures foreach of the other embodiments of the present invention discussed herein,there are two coordinate systems that define the measurement geometry.One is the coordinate axes of wafer 100 which are referred to as x, yand z (see FIG. 1b). The other is the coordinate axes of scan head 230relative to wafer 100 which is refereed to here as x', y' and z', whichare as indicated in FIG. 2.

As shown in FIG. 2, the x' axis is horizontal and in the plane of thefigure, the z' axis is vertical and in the plane of the figure, and they' axis (the axis of the measurement) is perpendicular to and into theplane of the figure. Thus, in this specific example, the measurement isbeing made on right grating 30 (see FIG. 1b). Initially wafer 100 isplaced on stage 200 and stage 200 is then aligned by rotating stage 200so that the x- and y-directions of gratings 20 and 30 on wafer 100 aresubstantially parallel to x' and y' axes of scan head 230. If the twoaxes systems are not exactly aligned then an imaginary line drawnbetween illuminated spots 250a and 250b will not be parallel to the axisthat is not being measured on wafer 10 (see FIG. 3 where the axis notbeing measured is the x-axis) and one spot will be slightly advancedalong the y-axis with respect to the other in the grating pattern beingused to measure alignment.

The optical part of the system of FIG. 2 incorporated within scan head230 includes light source 140 (e.g., a diode laser, or any coherent orincoherent light source) that directs a beam of light 300 tobirefringent polarizing prism 130 (e.g., a Rochon prism) where the lightis split into two light beams 210a and 210b, which are linearly andorthogonally polarized upon emergence from the prism. The prism alsointroduces a small angular deviation between the direction ofpropagation of the two beams, which is determined by the prism design.

Light beams 210a and 210b in turn are directed through non-polarizingfirst beam splitter 120 that is designed to transmit and reflect lightin about equal parts with the two transmitted light beams directed tolens 110 (e.g., a single element or multiple element lens) where the twotransmitted light beams are focused by lens 110 at spots 250a and 250b,on the right grating 30 on wafer 100, as shown in FIG. 3.

The reflected light from each of spots 250a and 250b on wafer 100 isthen collected by lens 110, impinges on the first beam splitter 120where the light is reflected through substantially 90° to a second beamsplitter 150 which is of a polarizing type. Since the reflected lightfrom each of spots 250a and 250b is essentially preserved in thepolarization state, beamsplitter 150 separates the light intotransmitted light from one spot, and reflected light from the other.Thus, light reflected from spot 250a is shown here passing throughsecond beam splitter 150 to lens 160a where it is focused on detector170a. Similarly, the light from spot 250b is reflected throughapproximately 90° by second beam splitter 150 to lens 160b and impingeson detector 170b.

Each of light detectors 170a and 170b (e.g., silicon diode detectors orphoto-multipliers), in-turn generates an analog electrical signal thatis proportional to the intensity of the light received by the respectivedetector. That analog electrical signal from each of detectors 170a and170b is applied to an optional corresponding AID converter(analog-to-digital converter) 180a and 180b, respectively, with eachresulting digital signal being applied to computer 190. The offsetbetween the two signals is then determined by computer 190 as discussedbelow in relation to FIG. 4, with that offset being directlyproportional to the misalignment between the portions of the gratingpattern on each of the consecutive layers of wafer 100 in the directionin which the measurement was made.

The measurement accuracy is dependent on the intervals at which thesignals are sampled by the A/D converters. The sampling interval S(i.e., the distance the spot moves between consecutive samples, in unitsof length) is calculated as: ##EQU1## Typically, with a scan speed of 10mm/sec, and digitizing frequency of 1000 samples/sec, the samplinginterval is 10 nm with the measurement accuracy getting better as thesampling interval decreases.

To initially focus the light beams on spots 250a and 250b, scan head 230can be moved in the z-direction under the control of computer 190 byfocus actuator 260 to physically raise and lower scan head 230 asnecessary. Also, to measure the x-axis of wafer 100, a second opticalsystem could be employed, wafer 100 could be rotated 90° relative tolight beams 250a and 250b, or scan head 230 could be rotated through90°, and the second measurement along the x'-axis made using leftgrating 20 in the same way as described above for the y'-axis.Typically, scan head 230 is moved with respect to wafer 100 rather thanmoving stage 200 because the optical scan head can be made to be muchsmaller and lighter than x-y stage 200.

There are several design considerations that are worth noting. First isthat prism 130 polarizes light beam 300 as it is split into two lightbeams, 210a and 210b. If the incoming light to prism 130 is polarized ina direction different than 45°, the intensity of the two outgoing beamswill not be equal, and will vary if the orientation of the polarizationof the incoming beam changes. For this reason, circularly polarizedlight, or un-polarized light in beam 300, is preferred, as it ensuresessentially equal energy in the two outgoing beams, 210a and 210b.

Also, it is desired to have a spot size (defined by the first zeroes ofthe diffraction limited irradiance pattern) that is about half theperiod of the gratings (i.e., the line width in alignment pattern 10) sothat the modulation of the light by the grating has the highestcontrast. The spot size is related to the numerical aperture (NA) of thelens by a fundamental, well known relationship: D=kλNA, where k is aconstant between 0.6 to 1, λ is the wavelength. This considerationdefines the desired numerical aperture of the lens.

The waveforms that are generated by the measurement process describedabove are presented schematically in FIG. 4. The waveform 310acorresponds to the output of the digitizer 180a (FIG. 2), and thewaveform 310b corresponds to the output of digitizer 180b (FIG. 2). Thevertical axis in FIG. 4 represents the magnitude of the detected light,and the horizontal axis represents elapsed time. Since the scan rate isessentially constant, the elapsed time is proportional to the scandistance, so that the horizontal axis also represents position in thescan direction.

To illustrate how the misalignment between the two layers on wafer 100is determined, waveforms 310a and 310b in FIG. 4 are drawn for such amisalignment, as well as an offset between the axes of wafer 100 (x, yand z) and the axes of scan head 230 (x', y' and z').

The following discussion requires reference to both FIGS. 3 and 4simultaneously. In FIG. 4 waveforms 310a and 310b are shown in relationto each other as scan head 230 is advanced across wafer 100 (hereoriented along the y-axis). Those waveforms are all shown divided intothree segments 330, 340 and 350. Segment 330 represents the signalsobtained by scanning lines 70a (see FIG. 3), segment 340 represents thesignals obtained by scanning lines 60b and 70b, and segment 350represents the signals obtained by scanning lines 60a.

The first segment 330 of signals 310a and 310b is a first calibrationsegment since both signals correspond to a time when both illuminationpoints, 250a and 250b impinge on lines 70a of target 30, as the scanhead is translated in the positive y-direction. In segment 330 therelationship of spots 250a and 250b with respect to the measurement axiscan be determined since there is no alignment error between lines 70a(i.e., the offset of the axes of the first layer of semiconductor wafer100 and scan head 230 can be determined with that portion of the gratingcontributed by the first layer of the semiconductor wafer).

The second segment 340 is a measurement segment since each of signals310a and 310b are contributed by the scanning of a portion of the twogratings contributed by the two layers of semiconductor wafer 100 (i.e.,spot 250a impinges on lines 60b of the second layer and spot 250bimpinges on lines 70b of the first layer).

The third segment 350 of signals 310a and 310b is a second calibrationsegment since both signals are obtained from lines 60a on a second layerof wafer 100 (i.e., both signals correspond to a time when bothillumination points, 250a and 250b impinge on lines 60a as scan head 230is translated in the positive y-direction). In segment 350 therelationship of spots 250a and 250b with respect to the measurement axiscan be determined since there is no alignment error between the lines60a (i.e., the offset of the axes of the second layer of semiconductorwafer 100 and scan head 230 can be determined with that portion of thegrating contributed by the second layer of the semiconductor wafer).

It should be noted that if patterns 60a, 60b and 70a, 70b are perfectlyparallel to the x'-axis of scan head 230, and furthermore ifillumination points 250a and 250b are on the identical y'-coordinate ofscan head 230, then waveforms 310a and 310b will be perfectly in phaseduring segments 330 and 350 since neither of illumination points 250aand 250b will be advanced ahead of the other (e.g., in FIG. 3illumination point 250b is shown advanced with respect to illuminationpoint 250a).

The calculations performed by computer 190 consist of a determination ofthe phase differences during the three segments 330, 340 and 350. Thephase differences during segments 330 and 350 are due to the previouslyexplained imperfect rotational alignment of the pattern on wafer 100 andthe axes of scan head 230 which produces the different coordinates ofillumination points 250a and 250b with respect to the actual axis ofwafer 100 in the direction that it is being scanned.

Then, the actual y-axis registration error between the two layers ofwafer 100 is the difference between the phase error between waveforms310a and 310b during segment 340, minus the phase error of the samewaveforms during calibration segments 330 and 350. Thus this two-beammeasurement method eliminates common mode errors.

Methods of computing the phase error, or time shift between twowaveforms are well established, including Fourier transform algorithms,zero-crossing detection and others, however cross-correlation algorithmsproduce excellent results with the present invention when a significantamount of noise is present. The cross-correlation of sampled waveformsis calculated for various integer displacements of the sampling intervalof waveforms 310a and 310b. The displacement where the sampledcross-correlations reach the peak is the phase difference, and isdetermined in segments 330, 340 and 350. The registration error iscalculated by D=P•φ, where P is the grating period, and φ is thecalibrated phase difference between the two signals, which is given by:φ=φ_(b) -0.5(φ_(a) +φ_(c)). The parameters of this equation are definedas:

φ_(a) =the phase difference between signals 310a and 310b duringinterval 330;

φ_(b) =the phase difference between the same signals during interval340; and

φ_(c) =the phase difference between the same signals during interval350.

In some cases it may be desirable to make several scans, either at thesame x-coordinate, or at different x-coordinates. Several scans aredesirable to reduce measurement noise since several measurements areaveraged, statistically random noise is reduced. Assuming that ascanning is in the y-axis direction (as described before and indicatedin the figure), it is desirable to move the scan line along the x-axisdirection so that localized imperfections in the grating structure(which are statistically random) can be averaged. For each scan theregistration error is computed in each of segments 330 and 350, and thenan average is taken of those values. Computing the registration errorand then averaging the registration errors is preferable to averagingthe intensity values of each sample point since errors due to vibrationcancel out when the samples are taken within a small part of a singlecycle of the predominant vibration frequency. From experience, thesevibration frequencies when present are typically well below 500 hertz.

The embodiment of FIG. 2 uses an optical head 230 that is movedmechanically by scan actuator 240 so that the light beams scan acrossthe gratings, and as discussed above, one of the technics that can beused to scan wafer 100 in the x' direction is to rotate scan head 230through 90° and repeat the process described above for the y' axis. Incertain applications, however, it may be advantageous to perform thescan without rotating the scan head, or perhaps without having to moveeither the scan head or the wafer. One way to perform that function isthrough the use of scanning mirrors (i.e, mirrors that are rotatedthrough a small angle to scan the light beam across the surface of thewafer in the direction of the desired scan).

Referring now to FIG. 7a where a rotating mirror modification is shownfor the embodiment of FIG. 2. In FIG. 7a a side view of scan head 230'"of FIG. 2 is shown where in this view the y'-axis is horizontal in theplane of the figure, while the x' axis is into the plane of the figure,and the z' axis is vertical as it is in FIG. 2. The components that arethe same as in FIG. 2, function in the same way as discussed above withrespect to FIG. 2, are shown here with the same reference numbers.Beamsplitter 120 in FIG. 7a is rotated 90° relative to its orientationin FIG. 2 allowing all of the instrument elements to be shown. Botharrangements (FIGS. 2 and 7a) are correct, as the choice of thedirection of the return beams (determined by the orientation ofbeamsplitter 120) is not material and is usually determined only byconvenience.

There are several additional components shown in FIG. 7a that were notshown in FIG. 2. In this embodiment rather than moving scan head 230'"or stage 200 in the direction of the y' axis, the light beams to andfrom wafer 100, from and to beam splitter 120 are reflected by mirror610. Additionally, mirror 610 is rotated through a small angle by scancontrol 630 (e.g., a servo), about an axis of rotation that is parallelto the x'-axis, under control of computer 190. That rotation of mirror610 thus causes the two light beams from beam splitter 120 to scanacross wafer 100 in the direction of the y' axis.

The embodiment of FIG. 7a can also be modified to operate with only onebeam, as illustrated in FIG. 7b, where light from source 140 is scannedby mirror 610 along the measurement axis y' (as before). By rotatingmirror 610 in the orthogonal direction, x', the beam can be directedalternately from one scan path to the other (e.g., the scan paths ofbeams 250a and 250b in FIG. 3). Since only one beam is used, there isonly one detector (170) to produce the signals. Compared to the two beamembodiment, the signal processing requires an additional step: the timedelay between the consecutive alternate scans needs to be subtractedfrom the second scan time axis, so that the signals can be representedon the same time axis as in FIG. 4. This single beam scanning does nothave the advantage of common mode rejection of vibration induced errors.

There are other well-known optical scanning methods, such as thosepresented in the literature (See Laser Beam Scanning by Gerald F.Marshall, Marcel Dekker, Inc., 1985) and include rotating prisms,rotating multifaceted mirrors and acousto-optic deflectors. Any of thosemay be adapted to perform similarly to the performance discussed withrelation to FIGS. 7a and 7b.

The previously described embodiments of FIGS. 2, 7a and 7b each use alinear scan (i.e., the beams are moved at a constant velocity). In someapplications it may be advantageous to employ a varying velocity scan.In particular, a sinusoidal velocity scan may be used, because it iseasily generated by driving the scan mechanism in an oscillatingfashion. The resulting signals are frequency modulated by the sinusoidalscan. The implementation of the sinusoidal scan does not require anymodification to the apparatus as described in FIG. 2 or FIG. 7a, it isdifferent only in that computer 190 is programmed to produce sinusoidalscan commands rather than linear to control scan actuator 240.

Additionally, there are well known signal processing methods forextracting relative phase information from frequency modulated signals.One approach that uses the ratio of the amplitude of the fundamental andfirst harmonic frequency components of the signal to measure the phaseis described by Noah Bareket and Wayne W. Metheny in "Phase ModulatedPolarizing Interferometer" U.S. Pat. No. 4,480,916.

Further, there are alternative methods for generating the two spotillumination 250a and 250b that may be advantageous in certainapplications. The advantage may be in lower implementation cost, and isapplication dependent.

One such alternative is illustrated in FIG. 6b which incorporates adiffraction grating to split the light into the required two beams. Inthis embodiment diffraction grating 135 replaces prism 130 of theembodiment in FIG. 2. The grating 135 is composed of equally spacedlines of opaque coating on a glass substrate, creating a common opticalelement that is known as a Ronchi Ruling. A discussion of the propertiesof such gratings can be found in Modern Optical Engineering by Warren J.Smith, McGraw-Hill, 1990, page 154. The first diffraction orders areseparated by an angle a, given by the equation sin α=2λ/S, where λ isthe illumination wavelength and S is the grating period. The two firstdiffraction orders are used to provide the two illumination beams 210aand 210b. As before, the reflected light from wafer 100 is directed bybeam splitter 120 toward detector 175. To separate the two beams theyare imaged by lens 165 on detector 175, which comprises two detectingelements, 175a and 175b, as shown in FIG. 6b. The signals of eachelement are digitized by the corresponding A/D converter (180a and180b), and acquired by computer 190 as previously discussed. Thenon-diffracted zero order light is focused in between the detectorelements and does not interfere with the measurement.

The embodiment of FIG. 5a incorporates a time division multiplexingapproach in scan head 230' where illumination points 250a and 250b arealternately illuminated. Here scan head 230' includes pulse generator420 that controls lasing diodes 410a and 410b located symmetricallyabout the optical axis and on approximately the same y'-coordinate ofscan head 230'. To alternatively power lasing diodes 410a and 410b,pulse generator 420 produces reciprocal square waves 445a and 445b(i.e., 180° out of phase with each other) as shown in FIG. 5b. Thepulsing light beams from each of lasing diodes 410a and 410b, in turn,are directed to substantially the same point on opposite sides of beamsplitter 430 where the two light beams are separated and directed towardillumination points 250a and 250b on wafer 100. The two separatedpulsing light beams from beam splitter 430 are then directed to beamsplitter 460 where some of the light from each of the two beams isreflected towards lens 440 which focuses that light on detector 450,while the balance of the light of each beam passes through beam splitter460 and proceeds to objective lens 110 and then to illumination spots250a and 250b on wafer 100 as in the first embodiment discussed inrelation to FIG. 2 (i.e., since the two light beams are pulsing 180° outof phase from each other, illumination points 250a and 250b in thisembodiment are not both illuminated at the same time). The lightreflected from each of illumination points 250a and 250b on wafer 100 inturn is reflected back through lens 110 to beam splitter 460 and isdeflected to lens 470 where the light is focused on detector 480.

The analog signals that are generated by detectors 450 and 480 areapplied to A/D converters 490b and 490a, respectively. Each of detectors490a and 490b are strobed by pulse train 495 (see FIG. 5b), which isdouble the frequency of and synchronized with pulse trains 445a and445b, under control of computer 190. The strobing of A/D converters 490aand 490b with waveform 495 separates the data contributed by each of thepulsed light beams produced by pulsed lasing diodes 410a and 410b beforethat data is handed off to computer 190.

In this configuration detector 450 detects the light waveforms directedtoward wafer 100, while detector 470 detects the light waveformsreflected from wafer 100. With the data from A/D converter 490b used foroptimization by normalizing the values derived from A/D converter 490a(i.e., to compensate for a variation of intensity of laser diodes 410aand 410b) thus, theoretically, lens 440, detector 450 and A/D converter490b could have been omitted.

FIG. 6a illustrates another embodiment of the present invention thatuses two light sources and spectral multiplexing. Since the opticalsystem of scan head 230" for this technic is quite similar to theapproach of FIG. 2, the components that are the same and which functionin the same way have the same reference numbers as in FIG. 2. In thisembodiment the illumination sources are lasing diodes 510a and 510b thatemit light of a different wave length from the other so that the lightreflected from wafer 100 can be separated by virtue of the differencesin wavelength. In the optical path, beam splitter 430 combines the lightfrom the two sources, which are oriented so that the combined beamspropagate with the required angular separation to produce the twoilluminating beams 210a and 210b.

In FIG. 6a the light reflected from wafer 100 and then reflected by beamsplitter 120 is directed to a third beam splitter 520 which is of adichroic type that transmits light of one wavelength (corresponding tothe wavelength of one laser source), and reflects light of the otherwavelength. The light from the spots 250a and 250b are thus separated byvirtue of the difference in wavelength. From this point on theembodiment of FIG. 6a functions in an identical way to the embodiment ofFIG. 2. This portion of the embodiment of FIG. 6a being substantiallythe same as that of FIG. 2, causes the calculation of the misalignmentbetween the two layers on wafer 100 to be calculated in the same way asdescribed with respect to FIG. 2.

Each of the embodiments discussed above use light for the measurement,however, in certain circumstances it can be advantageous to perform themeasurement with another form of radiation, or an atomic scalemicroscope. In particular, another radiation approach that is mostsuitable is a scanning electron microscope, where the scanning electronbeam can be programmed to scan the target 10 on wafer 100. Any one ofthe many commercially available scanning electron microscopes (e.g., KLAModel 8100) can perform the measurement without modification. In such animplementation, the microscope is programmed to scan the two portions ofthe target on the different semiconductor layers alternatively. Thecomputer system of the microscope then stores the signals obtained byscanning, which are essentially similar to the signals obtained by thepreviously described embodiments, and once the signals are acquired,they are processed by the computer in essentially the same way as in theprior embodiments. Because the scan speed of an electron microscope isvery high, the accuracy of the measurement is not degraded by anyvibrations that might be present even though the two scans are performedserially.

FIG. 8 is a schematic representation of a scanning electron-beamimplementation of the present invention. Included within vacuum chamber760 is scanning electron-beam microscope 7 within an electron opticalcolumn 17 above wafer 100 on stage 200 which is located within lowerchamber 17a. In the simplified schematic of the electron-beam microscope7 shown here is electron source 13 that is connected to external voltagesource 11 with a narrow beam of highly accelerated electrons 14emanating from electron source 13 that is directed toward wafer 100.

In the path between electron source 13 and wafer 100 are a plurality ofelectron lens L₁, L₂ and L₃ to focus the electron beam on wafer 100.Intermediate lens L₂ and L₃ there is also shown an electrostaticdeflection system to selectively scan electron beam 14 across thesurface of wafer 100. As shown here the deflection system includes fourpairs of electron beam scanning coils D₁, D₂, D₃ and D₄. With deflectioncoils D₁, D₂, D₃ and D₄, beam 14 can be deflected to serially scanacross the surface of wafer 100 in selected x axis and y axis paths forthe purposes of the present invention to gather data from beams 250a and250b (in each of the x and y axis scans) as in FIG. 3. In turn,deflection coils D₁, D₂, D₃ and D₄ are controlled by computer 190 viadigital-to-analog (D/A) converters 780a and 780b and saw-toothgenerators 19 and 21.

As beam 14 is scanned across wafer 100, and target 30 (not shown)thereon, secondary and backscatter electrons from wafer 100 are detectedby electron collector 25. The signal made up of the detected electronsfrom electron collector 25 is then applied to amplifier 23 and then tocomputer 190 via A/D converter 180. Thus, as in the other embodimentsdiscussed above where the scans are performed serially, computer 190analysis the data here in the same way. Of the serial scanningembodiments of the present invention, the scanning electron microscopeembodiment just discussed is superior since it is very fast andtherefore less likely to be impacted by vibrations which might have aserious impact on the accuracy of a traditional optical embodiment.

Other embodiments that might be used are those that are able tomechanically detect the microscopic differences in height of features ona wafer. Some devices having that capability are Atomic ForceMicroscopes (ATM), Scanning Tunneling Microscopes (STM), Near-FiledOptical Microscopes, etc. This general field of the art was the topic ofa conference on Scanning Probe Microscopy by the American Institute ofPhysics at Santa Barbara, Calif., in 1991 recorded in the API ConferenceProceedings 241, "Scanning Probe Microscopy", H. Kumar Wickramasinghe,Editor.

FIG. 9 illustrates how those techniques can be employed for the presentinvention in simplified schematic form. Wafer 100 to be scanned in shownon stage 200 with the positioning stage 200 under control of computer190. The scanning probe hardware, including the necessary supportelectronics is shown as block 730 with probe 720 in communication withblock 730. Thus, as probe 720 scans target 20 or 30 (not shown--see FIG.2) on wafer 100 as stage 200 is translated in either the x or ydirection, excursions of probe 720 in the z direction produce an analogelectronic signal in the electronics within block 730 which is convertedto a digital signal by A/D converter 180 and then applied to computer190. Thus it can be seen that the signal produced by the mechanicalmotion of probe 720 in the z direction is directly proportional to theoptically detected differences in the previously discussed embodimentsand therefore is processed similarly by computer 190. Here also the userhas the option of using multiple probes simultaneously as in theprevious embodiments to have the effect of beams 250a and 250b, or ofperforming the scans serially as with the scanning electron microscope.

This embodiment is shown in somewhat more detail in FIGS. 10a and 10bwith piezo driven cantilever 710 with an atomic sized tip 720 (e.g.,single crystal of silicon). Here, as tip 720 is traverses across targetpattern 20 on wafer 100 (as either wafer 100 is translated by computer190 on stage 200, or as cantilever 710 is drawn across wafer 100), tip720 is translated in the z direction by the lines and spaces between thelines of target 20. That vertical translation of tip 720 causescantilever 710 to applied atomic force changes to the piezo crystal inpiezo electric detector 700 which generates a varying electrical signalby the piezo crystal. That electrical signal is them applied toamplifier 23' and then to computer 190 where the analysis is the same asin other embodiments discussed above.

While this invention has been described in several modes of operationand with exemplary routines and apparatus, it is contemplated thatpersons skilled in the art, upon reading the preceding description andstudying the drawings, will realize various alternatives approaches tothe implementation of the present invention. It is therefore intendedthat the following appended claims be interpreted as including all suchalterations and modifications that fall within the true spirit and scopeof the present invention.

What is claimed:
 1. A target for use in measuring the relative positionbetween two substantially coplanar layers of a device wherein a secondlayer of said two layers is located on a first layer of said two layers,said target comprising:a first periodic structure to be placed on saidfirst layer of said device that is visible through said second layer;and a second periodic structure that complements said first periodicstructure with said second periodic structure placed on said secondlayer of said device at a location that is adjacent said first periodicstructure when said second layer is placed on said first layer with saidfirst and second layers being in fixed position with respect to eachother.
 2. The target of claim 1 wherein said second periodic structureis the same said first periodic structure rotated through 180°.
 3. Thetarget of claim 2 wherein each of said first and second periodicstructures are substantially in the shape of the letter L.
 4. The targetof claim 1 wherein:each of said first and second periodic structureshave a first section of a first selected length, and a second section ofa second selected length with said second selected length being up toone-half said first selected length; and each of said second sections ofeach of said first and second periodic structures is disposed to beadjacent a portion of each of said first section and said second sectionof the other one of said first and second periodic structures.
 5. Thetarget of claim 4 wherein:said first section of each of said first andsecond periodic structures includes at least two adjacent first linesegments each having a substantially uniform thickness, m, and a lengththat is substantially equal to said first selected length, with each ofsaid first line segments being separated from an adjacent first linesegment by a substantially uniform distance n; and said second sectionof each of said first and second periodic structures includes at leasttwo adjacent second line segments each having a substantially uniformthickness, m, and a length that is substantially equal to said secondselected length, with each of said second line segments being separatedfrom an adjacent second line segment by a substantially uniform distancen; wherein, for each of said first and second periodic structures, aline of said first section is separated from a line of saidcorresponding second section by said substantially uniform distance n.6. The target of claim 5 wherein said thickness m and distance n have afixed ratio relationship with respect to each other.
 7. The target ofclaim 5 wherein said thickness m and distance n are substantially equalto each other.
 8. The target of claim 5 wherein:each of said linesegments of each of said first and second sections has a first end and asecond end; in said first periodic structure the first end of each linesegment in each of said first and second sections are substantiallyaligned with each other; and in said second periodic structure thesecond end of each line segment in said first section are substantiallyaligned with the second end of each line segment in said second section.9. The target of claim 4 wherein said first and second periodicstructures are adjacent each other without any portion of one touchingor interleaving with any portion of the other.
 10. The target of claim 4wherein:said first section of each of said first and second periodicstructures is provided for measurement calibration of a correspondingone of said first and second layers of said device on which said firstsection is located; and said second section of each of said first andsecond periodic structure is provided for a measurement of the relativeposition between said two substantially coplanar layers of said deviceusing both of said second sections.
 11. An apparatus to measure therelative position between two substantially coplanar layers of a devicewherein a second layer of said two layers is located on a first layer ofsaid two layers utilizing a target including a first periodic structureon said first layer of said device that is visible through said secondlayer and a second periodic structure that complements said firstperiodic structure with said second periodic structure on said secondlayer of said device at a location that is adjacent, and in a fixedposition with respect to, said first periodic structure, said apparatuscomprising:a optical scan head positioned relative to said device togather optical data from said target, said optical scan head including:alight source to provide a light beam; a birefringent polarizing prism toreceive said light beam from said light source and to split said lightbeam into first and second linearly and orthogonally polarized lightbeams, each of said first and second light beams having a selectedangular deviation from the other of said first and second polarizedlight beam; a non-polarizing beam splitter to transmit said first andsecond polarized light beams from said birefringent polarizing prism tophysically separated locations on said target, and to independentlyreceive and selectively direct each of a first and a second reflectedpolarized light beam from said separate locations on said target whereineach of said first and second reflected polarized light beamscorresponds to said first and second polarized light beams,respectively; and a detector positioned to independently receive each ofsaid first and second reflected polarized light beams from saidnon-polarizing beam splitter to independently generate a first and asecond electrical signal proportional to the intensity of thecorresponding one of said first and second reflected polarized lightbeams; and a computing and control system coupled to said optical scanhead and said device to provide movement therebetween to cause saidfirst and second polarized light beams from said non-polarizing beamsplitter to scan physically separated paths across said target, andcoupled to said detector to receive said signals therefrom to calculateany offset between said first periodic structure and said secondperiodic structure of said target.
 12. The apparatus of claim 11wherein:said optical scan head further includes a polarizing beamsplitter to receive said first and second reflected polarized lightbeams directed thereto by said non-polarizing beam splitter, and toseparate said first and second reflected polarized light beams bypassing said first reflected polarized light beam therethrough andreflecting said second reflected polarized light beam into another path;and said detector includes:a first detection device positioned toreceive said first reflected polarized light beam from said polarizingbeam splitter to generate an electrical signal proportional to theintensity of said first reflected polarized light beam; and a seconddetection device positioned to receive said second reflected polarizedlight beam from said polarizing beam splitter to generate an electricalsignal proportional to the intensity of said second reflected polarizedlight beam.
 13. The apparatus of claim 12 further includes:a first lensbetween said non-polarizing beam splitter and said device to focus saidfirst and second polarized light beams onto said target; a second lensbetween said polarizing beam splitter and said first detection device tofocus said first reflected polarized light beam onto said firstdetection device; and a third lens between said polarizing beam splitterand said second detection device to focus said second reflectedpolarized light beam onto said second detection device.
 14. Theapparatus of claim 12 further includes:a first A/D converter betweensaid first detection device and said computing and control system; and asecond A/D converter between said second detection device and saidcomputing and control systems.
 15. The apparatus as in claim 11 whereinsaid light source produces circularly polarized light.
 16. The apparatusas in claim 11 wherein said light source produces un-polarized light.17. The apparatus of claim 11 wherein:each of said first and secondperiodic structures of said target include line segments having asubstantially uniform thickness and spacing between each of said lines;and each of said first and second polarized light beams directed to saidtarget from said non-polarizing beam splitter have a maximum spot sizeof one-half said thickness of said line segments within said target. 18.An apparatus to measure the relative position between two substantiallycoplanar layers of a device wherein a second layer of said two layers islocated on a first layer of said two layers utilizing a target includinga first periodic structure on said first layer of said device that isvisible through said second layer and a second periodic structure thatcomplements said first periodic structure with said second periodicstructure on said second layer of said device at a location that isadjacent, and in a fixed position with respect to, said first periodicstructure, said apparatus comprising:an optical scan head positionedrelative to said device to gather optical data from said target, saidoptical scan head including:a light source to provide a light beam; abeam splitter to transmit said light beam from said light source to saidtarget, and to receive and selectively direct a reflected light beamfrom said target wherein said reflected light beam corresponds to saidlight beam directed to said target; a mirror selectively rotatable aboutan axis wherein said axis has a selected angular relationship to a topsurface of said device to reflect said light beam from said beamsplitter to said target, and to direct said reflected light beam fromsaid target to said beam splitter; a scan transducer coupled to saidmirror to rotate said mirror about said axis through a selected angularrange to cause said light beam to scan across said target in a selectedscan path, and to position said axis of said mirror to select betweentwo angular relationships between said axis and said top surface of saiddevice to provide the capability to cause said light beam to scanthrough two physically separated scan paths each being related directlyto said angular relationship between said axis of said mirror and saidsurface of said device; and a detector positioned to receive saidreflected light beam from said beam splitter to generate an electricalsignal proportional to the intensity of said reflected light beam; amemory coupled to said detector to store scan signals for comparisonwith later received scan signals produced when said axis of said mirroris in another position relative to said device to calculate any offsetbetween said first and second periodic structures of said target; and acomputing and control system coupled to said scan transducer to causesaid light beam to scan across said target and to sequentially vary theangular relationship between said mirror axis and said surface of saiddevice in two such angular relationships, and coupled to receive signalsfrom said detector and said memory to calculate any offset between saidfirst periodic structure and said second periodic structure of saidtarget with two scan signals, one from each of said detector and saidmemory.
 19. An apparatus to measure the relative position between twosubstantially coplanar layers of a device wherein a second layer of saidtwo layers is located on a first layer of said two layers utilizing atarget including a first periodic structure on said first layer of saiddevice that is visible through said second layer and a second periodicstructure that complements said first periodic structure with saidsecond periodic structure on said second layer of said device at alocation that is adjacent, and in a fixed position with respect to, saidfirst periodic structure, said apparatus comprising:an optical scan headpositioned relative to said device to gather optical data from saidtarget, said optical scan head including:a light source to provide alight beam; a birefringent polarizing prism to receive said light beamfrom said light source and to split said light beam into first andsecond linearly and orthogonally polarized light beams, each of saidfirst and second light beams having a selected angular deviation fromthe other; a non-polarizing beam splitter to transmit said first andsecond polarized light beams from said polarizing prism to physicallyseparate locations on said target, and to independently receive andselectively direct each of a first and a second reflected polarizedlight beam from said target with each of said first and second reflectedpolarized light beams corresponding to said first and second polarizedlight beams, respectively; a selectively rotatable mirror to reflecteach of said first and second polarized light beams from saidnon-polarizing beam splitter through a selected angle to said target,and to independently direct each of said first and second reflectedlight beams from said target to said non-polarizing beam splitter; ascan transducer coupled to said mirror to rotate said mirror through aselected range of angles to cause each of said first and secondpolarized light beams to scan across said target in two scan paths;apolarizing beam splitter to receive each of said first and secondreflected polarized light beams directed thereto by said non-polarizingbeam splitter, and to separate the paths of said first and secondreflected polarized light beams by passing said first reflectedpolarized light beam therethrough and reflecting said second reflectedpolarized light beam into another path; a first detector positioned toreceive said first reflected polarized light beam from said polarizingbeam splitter to generate a first electrical signal proportional to theintensity of said first reflected polarized light beam; and a seconddetector positioned to receive said second reflected polarized lightbeam from said polarizing beam splitter to generate a second electricalsignal proportional to the intensity of said second reflected polarizedlight beam; and a computing and control system coupled to said scantransducer to cause each of said first and second polarized light beamsto scan across said target, and coupled to receive signals from saidfirst and second detectors to calculate any offset between said firstperiodic structure and said second periodic structure of said target.20. An apparatus to measure the relative position between twosubstantially coplanar layers of a device wherein a second layer of saidtwo layers is located on a first layer of said two layers utilizing atarget including a first periodic structure on said first layer of saiddevice that is visible through said second layer and a second periodicstructure that complements said first periodic structure with saidsecond periodic structure on said second layer of said device at alocation that is adjacent, and in a fixed position with respect to, saidfirst periodic structure, said apparatus comprising:an optical scan headpositioned relative to said device to gather optical data from saidtarget, said optical scan head including:a light source to provide alight beam; an optical grating to receive said light beam from saidlight source and to split said light beam into first and second lightbeams with a selected angular deviation between said first and secondlight beams; a beam splitter to transmit said first and second lightbeams from said optical grating to physically separate locations on saidtarget, and to receive and selectively direct each of a first and asecond reflected light beam from said target wherein each of said firstand second reflected light beams corresponds to said first and secondlight beams, respectively; and a detector positioned to independentlyreceive said first and second reflected light beams from said beamsplitter and to independently generate a first and a second electricalsignal proportional to the intensity of the corresponding one of saidfirst and second reflected light beams; and a computing and controlsystem coupled to said optical scan head and said device to providemovement therebetween to cause said first and second light beams fromsaid beam splitter to scan across said target, and coupled to saiddetector to receive said signals therefrom to calculate any offsetbetween said first periodic structure and said second periodic structureof said target.
 21. An apparatus to measure the relative positionbetween two substantially coplanar layers of a device wherein a secondlayer of said two layers is located on a first layer of said two layersutilizing a target including a first periodic structure on said firstlayer of said device that is visible through said second layer and asecond periodic structure that complements said first periodic structurewith said second periodic structure on said second layer of said deviceat a location that is adjacent, and in a fixed position with respect to,said first periodic structure, said apparatus comprising:an optical scanhead positioned relative to said device to gather optical data from saidtarget, said optical scan head including:first and second light sources;a pulse generator coupled to each of said first and second light sourcesto alternately ignite each of said first and second light sources at aselected frequency to provide first and second light beams ofsubstantially the same intensity 180° out of phase from each other; afirst beam splitter to which each of said first and second light beamsfrom said first and second light sources, respectively, are directed tosubstantially the same point on opposite sides of said first beamsplitter to separate said first and second light beams into separatepaths with each directed to a different point on said target; a secondbeam splitter having a first and a second surface thereof, to receive onsaid first surface each of said first and second light beams from saidfirst beam splitter at different points thereon to be transmittedtherethrough toward said target, and to receive on, and reflect from,said second surface a first and a second reflected light beam reflectedfrom said target in a selected direction wherein said first and secondreflected light beams correspond to said first and second light beams,respectively; and a first detector positioned to alternately receivesaid first and second reflected light beams from said second surface ofsaid second beam splitter and to alternately and independently generatea first and a second electrical signal proportional to the intensity ofsaid first and second reflected light beams, respectively; and acomputing and control system is coupled to said optical scan head andsaid device to provide movement therebetween to cause said first andsecond light beams from said second beam splitter to scan across saidtarget in physically separated paths; and coupled to, and strobes attwice said selected frequency and synchronized with said selectedfrequency, said first detector to receive said signals therefrom tocalculate any offset between said first periodic structure and saidsecond periodic structure of said target.
 22. The apparatus of claim 21wherein:a portion of each of said first and second light beams incidenton said second beam splitter are reflected from said first surface ofsaid second beam splitter; said optical scan head further includes asecond detector positioned to alternately receive said portion of saidfirst and second light beams reflected from said first surface of saidsecond beam splitter to alternately and independently generate a thirdand a fourth electrical signal proportional to the intensity of saidportion of said first and second light beams reflected from said firstsurface of said second beam splitter, respectively; and said computingand control system is coupled to said second detector to receive saidthird and fourth electrical signals to correct said calculated offsetbetween the first and second periodic structures resulting from avariation in intensity and duration difference between said first andsecond light beams.
 23. The apparatus of claim 22, further including:afirst lens between said second beam splitter and said device to focussaid first and second light beams onto said target; a second lensbetween said second surface of said second beam splitter and said firstdetector to focus said first and second reflected light beams from saidsecond surface of said second beam splitter onto said first detector;and a third lens between said first surface of said second beam splitterand said second detector to focus said first and second reflected lightbeams from said first surface of said second beam splitter onto saidsecond detector.
 24. The apparatus of claim 21 wherein said pulsegenerator produces first and second pulse trains that are 180° out ofphase with each other and at said selected frequency and with each ofsaid first and second pulse trains applied to said first and a secondlight sources to provide said first and a second light beams of the samerelative intensity that are 180° out of phase from each other.
 25. Anapparatus to measure the relative position between two substantiallycoplanar layers of a device wherein a second layer of said two layers islocated on a first layer of said two layers utilizing a target includinga first periodic structure on said first layer of said device that isvisible through said second layer and a second periodic structure thatcomplements said first periodic structure with said second periodicstructure on said second layer of said device at a location that isadjacent, and in a fixed position with respect to, said first periodicstructure, said apparatus comprising:an optical scan head positionedrelative to said device to gather optical data from said target, saidoptical scan head including:a spectral multiplexed light sourceincluding a first and a second light source providing a first and asecond light beam, respectively, each of a different wavelength and asame relative intensity; a first beam splitter having a first and asecond side with each of said first and second light beams directed tosubstantially the same point on said first and second sides of saidfirst beam splitter, respectively, to direct said first and second lightbeams into separate paths with each directed to different points on saidtarget; a second beam splitter having a first and a second surface withsaid first surface receiving and transmitting therethrough each of saidfirst and second light beams from said first beam splitter at differentpoints thereon to physically separate locations on said target, and saidsecond surface receiving and reflecting therefrom a first and a secondreflected light beam from said target wherein said first and secondreflected light beams correspond to said first and second light beams,respectively; a dichroic type beam splitter, being selected to passlight therethrough having a bandwidth that includes the wavelength ofthe light in said first light beam and excludes the wavelength of thelight in said second light beam, positioned to receive said first andsecond reflected light beams from said second surface of said secondbeam splitter, to pass said first reflected light beam therethrough, andto reflect said second reflected light beam therefrom in a differentdirection; a first detector positioned to receive said first reflectedlight beam passed through said dichroic type beam splitter to generate afirst electrical signal proportional to the intensity of said firstreflected light beam from said target; and a second detector positionedto receive said second reflected light beam from said dichroic type beamsplitter to generate a second electrical signal proportional to theintensity of said second reflected light beam from said target; and acomputing and control system coupled to said optical scan head and saiddevice to provide movement therebetween to cause said first and secondlight beams from said second beam splitter to scan across said target inphysically separated paths, and coupled to said first and seconddetectors to receive said signals therefrom to calculate any offsetbetween said first periodic structure and said second periodic structureof said target.
 26. The apparatus of claim 25 further includes:a firstlens between said second beam splitter and said device to focus saidfirst and second light beams onto said target; a second lens betweensaid dichroic type beam splitter and said first detector to focus saidfirst reflected light beam from said device onto said first detector;and a third lens between said dichroic type beam splitter and saidsecond detector to focus said second reflected light beam from saiddevice onto said second detector.
 27. The apparatus of claim 11, 18, 19,20, 21 or 25 wherein:each of said first and second periodic structureshave a first section of a first selected length, and a second section ofa second selected length with said second selected length being up toone-half said first selected length; each of said second sections ofeach of said first and second periodic structures is disposed to beadjacent a portion of each of said first section and said second sectionof the other one of said first and second periodic structures; saidfirst light beam is scanned sequentially across said first section ofsaid first periodic structure, said second section of said firstperiodic structure, and said first section of said second periodicstructure; and said second light beam is scanned sequentially acrosssaid first section of said first periodic structure, said second sectionof said second periodic structure, and said first section of said secondperiodic structure.
 28. A method for measuring the relative positionbetween two substantially coplanar layers of a device in a selecteddirection wherein a second layer of said two layers is located on afirst layer of said two layers, said method comprising the steps of:a.placing a first periodic structure of a first target on said first layerof said device oriented to facilitate measurement of said relativeposition between said two layers in said selected direction; b. placinga second periodic structure of said first target on said second layer,wherein said second periodic structure complements said first periodicstructure, at a location that is adjacent said first periodic structureand oriented similarly to the orientation of said first periodicstructure in step a. to facilitate measurement of said relative positionbetween said two layers in said selected direction with said firstperiodic structure being visible through said second layer, and saidfirst and second periodic structures in a fixed relationship withrespect to each other; c. scanning a radiation beam in a first pathacross portions of both said first and second periodic structures ofsteps a. and b.; d. scanning a radiation beam in a second path acrossportions of both said first and second periodic structures of steps a.and b. wherein said first and second paths are physically separated fromeach other; e. detecting radiation reflected from said first and secondperiodic structures in said first path as the scanning of step c. isperformed; f. detecting radiation reflected from said first and secondperiodic structures in said second path as the scanning of step d. isperformed; g. generating a first signal proportional to the intensity ofthe detected reflected radiation in step e.; h. generating a secondsignal proportional to the intensity of the detected reflected radiationin step f.; and i. calculating any offset between said first periodicstructure and said second periodic structure of said first target fromsaid signals generated in steps g. and h.
 29. The method of claim 28wherein:each of said first and second periodic structures of steps a.and b. have a first section of a first selected length, and a secondsection of a second selected length with said second selected lengthbeing less than one-half said first selected length; and each of saidsecond sections of each of said first and second periodic structures isdisposed to be adjacent a portion said first section and said secondsection of the other one of said first and second periodic structures.30. The method of claim 29 wherein:said first section of each of saidfirst and second periodic structures of steps a. and b. includes atleast two adjacent first line segments each having a substantiallyuniform thickness, m, and a length that is substantially equal to saidfirst selected length, with each of said first line segments beingseparated from an adjacent first line segment by a substantially uniformdistance n; and said second section of each of said first and secondperiodic structures includes at least two adjacent second line segmentseach having a substantially uniform thickness, m, and a length that issubstantially equal to said second selected length, with each of saidsecond line segments being separated from adjacent first and second linesegments of the same periodic structure by a substantially uniformdistance n;wherein, for each of said first and second periodicstructures, a line of said first section is separated from a line ofsaid corresponding second section by said substantially uniform distancen.
 31. The method of claim 30 wherein said thickness m and distance nhave a fixed ratio relationship to each other.
 32. The method of claim30 wherein said thickness m and distance n are substantially equal toeach other.
 33. The method of claim 30 wherein:each of said linesegments of each of said first and second sections has a first end and asecond end; in said first periodic structure the first end of each linesegment in each of said first and second sections are substantiallyaligned with each other; and in said second periodic structure thesecond end of each line segment in said first section are substantiallyaligned with the second end of each line segment in said second section.34. The method of claim 29:wherein step c. includes scanning saidradiation beam sequentially across said first section of said firstperiodic structure, said second section of said first periodicstructure, and said first section of said second periodic structure; andwherein step d. includes scanning said radiation beam sequentiallyacross said first section of said first periodic structure, said secondsection of said second periodic structure, and said first section ofsaid second periodic structure.
 35. The method of claim 29 wherein:saidfirst section of each of said first and second periodic structures isprovided for measurement calibration of a corresponding one of saidfirst and second layers of said device on which said first section islocated; and said second section of each of said first and secondperiodic structure is provided for a measurement of the relativeposition between said two substantially coplanar layers of said deviceusing both of said second sections.
 36. The method of claim 28 whereinthe scanning of steps c. and d. are substantially parallel to eachother.
 37. The method of claim 28 wherein the scanning of steps c. andd. are performed sequentially.
 38. The method of claim 28 wherein thescanning of steps c. and d. are performed simultaneously.
 39. The methodof claim 28 wherein:a first axis of interest is identified on thesurface of said device; and said first and second periodic structures ofsteps a. and b. are placed on said first and second layers of saiddevice, respectively, to facilitate measurement of the relative positionbetween said two layers in the direction of said first axis of interest.40. The method of claim 39:wherein a second axis of interest, at aselected angle to said first axis of interest, is identified on thesurface of said device; and said method further includes the steps of:j.placing a third periodic structure of a second target on said firstlayer of said device oriented to facilitate measurement of said relativeposition between said two layers in the direction of said second axis ofinterest; k. placing a fourth periodic structure of said second target,wherein said fourth periodic structure complements said third periodicstructure, at a location that is adjacent said third periodic structureand oriented similarly to the orientation of said third periodicstructure in step j. to facilitate determining said relative positionbetween said two layers in the direction of said second axis of interestwith said fourth periodic structure being visible through said secondlayer; l. scanning a radiation beam in a third path across portions ofboth said third and fourth periodic structures of steps j. and k.; m.scanning a radiation beam in a fourth path across portions of both saidthird and fourth periodic structures of steps j. and k. with said thirdand fourth paths physically separated form each other; n. detectingradiation reflected from said third and fourth periodic structures insaid third path as the scanning of step l. is performed; o. detectingradiation reflected from said third and fourth periodic structures insaid fourth path as the scanning of step m. is performed; p. generatinga third signal proportional to the intensity of the detected reflectedradiation in step n.; q. generating a fourth signal proportional to theintensity of the detected reflected radiation in step o.; and r.calculating any offset in the direction of said second axis of interestbetween said third periodic structure and said fourth periodic structureof said second target from said signals generated in steps p. and q. 41.The method of claim 40 wherein said first and second axes of interestare substantially perpendicular to each other.
 42. The method of claim28 wherein said scanning of steps c. and d. is performed optically. 43.The method of claim 28 wherein said scanning of steps c. and d. isperformed with an electron beam.
 44. The method of claim 28 wherein saidsecond periodic structure is the same as said first periodic structurerotated through 180°.
 45. The method of claim 44 wherein each of saidfirst and second periodic structures are substantially in the shape ofthe letter L.